Fuse circuits are typically utilized to permanently set a logic state through the use of a programmable fuse link. Fusible links are typically incorporated in a conductive path of a logic circuit such that when the fuse is intact, current flows therethrough to present one logic state on the output of the fuse circuit. When the fuse is broken or open, a resulting high impedance causes an interruption of current, thus forcing the output circuit associated with the fuse to a different logic state.
Typically, the fuse is a thin strip of conductive material that can be severed or broken by passing an excessive amount of current therethrough. This is achieved by attaching a programming pad to one side of the fuse, the other side being connected to reference voltage such as ground or V.sub.SS. The voltage on the pad is then raised to a predetermined programming voltage, above which excessive current passes through the fuse and causes it to "burn out" and therefore provide an open circuit.
One disadvantage to programming fuses in the conventional way is that if the voltage reaches a level that is higher than the power supply, a current path can be created from the programming pad back through active circuitry to the power supply and thus pump current through the power supply. This can result in damage to other active components. It is therefore desirable to have some type of isolation. Another disadvantage to present fuse circuits is that the only way to determine the state of the fuse is to actually program the fuse by opening it and then observing the output. Some systems have actually incorporated gate logic to allow a logic signal to be ORed with the output of the fuse circuit to determine the effect that the fuse will have on the circuit. For example, in some circuits, it is desirable to determine what effect a certain program word will have. This can be achieved by forcing the output of the fuse circuits to a predetermined logic state and then observing the effect of this output state on the rest of the circuitry. However, this requires additional logic, and if the fuse is faulty, an error may result.